Thermoplastic aromatic Polyether Ketone derivatives, such as Polyether Ether Ketone (PEEK), are well known to the art. These polymers have melting points greater than 330xc2x0 C., continuous use temperatures of 260xc2x0 C. or more and high mechanical strengths, such as tensile strength greater than 85 MPa. They have significant commercial utility as plastics, especially as molded articles and as composites with glass/carbon/Kevlar fibres for a variety of structural applications including in aerospace and general engineering industries. PEEK also finds applications as extruded rods and profiles for manufacture of bushings, seals, etc. In general, they are processed using extruders and injection molding machines in temperature range of 360-400xc2x0 C., thus requiring extremely high thermal stability.
Literature teaches us two major processes, nucleophilic and electrophilic, for the production of thermoplastic aromatic Polyether Ether Ketone. One is described by Johnson et al. (J. Polymer Sci. 5, A-1, 2371 1967). This nucleophilic route employs hydroquinone and dihalobenzophenone along with a base, in solvents like N-Methyl-Pyrrolidone or Sulfolane, at temperatures of about 200-250xc2x0 C. The PEEK so produced, however, is found to be of low molecular weight [Inherent Viscosity (Inh, V.) less than 0.7 dl/g] and cannot be used as a molded plastic due to it""s low mechanical properties.
An improvement on this product and process (U.S. Pat. No. 4,320,224/GB 1586 972), involving nucleophilic route is brought about by employing a high boiling solvent Diphenyl Sulfone. In this reaction hydroquinone is transformed into its di-potassium salt by heating with an equivalent amount of potassium carbonate or potassium bicarbonate, with simultaneous removal of the water at 150-200xc2x0 C., followed by addition of the second monomer, namely, 4,4xe2x80x2-difluoro benzopheone. The polymerization reaction is carried out at 320-350xc2x0 C. to obtain polymer of desired Inh. V. range of 0.8 to 1.4 dl/g with melting point of 335-350xc2x0 C. PEEK so produced has structure as well known in the art as given below with two fluoride end-groups.
This process is commercially utilised today. It, however, has several drawbacks. First, it uses expensive raw materials containing Fluorine and Potassium, both of which end up as a by-product to be separated from PEEK. It also requires use of very high temperature for organic reactions, like 300xc2x0 C. and above. The use of such high temperature also brings about some charring of material requiring special, melt filtration of the PEEK polymer to remove black specs formed during the manufacturing process. The formation of a stoichiometric amount of Potassium Fluoride as a by-product, requires elaborate salt separation procedures to obtain the polymer in pure form. The Diphenyl Sulfone solvent used has a high melting point of 129xc2x0 C., which makes it inconvenient to process it except at high temperatures. Diphenyl Sulfone is further immiscible with water, hence requiring use of non-aqueous systems for precipitation of the polymer, making its removal from the reaction mass cumbersome.
Hence a process of PEEK manufacturing which can be carried out at lower temperatures, where PEEK can be precipitated in water instead of non-aqueous non-solvents and where recycling of by-products is feasible is most desirable.
Another route for production of thermoplastic aromatic Polyether Ketones like PEEK, involves use of Friedel-Crafts catalysts (electrophilic process). For example, European Patent No. 0174207 teaches the use of AlCl3 for the polymerization of a carboxylic acid chloride derivative of Phenoxy Benzoic Acid (PBA) and Phenoxy Phenoxy Benzoic Acid (PPBA) to give Polyether Ketone (PEK) and Polyether Ether Ketone (PEEK) respectively. The process, though carried out at low temperatures such as 0-30xc2x0 C., uses AlCl3 in CH2Cl2 solution. Due to the heterogeneous nature of this reaction, generally undesirable lower molecular weight polymers are produced. PEEK polymer obtained by this process is, also, predominantly non-linear and show a high degree of branching. These defects lead to a lowering of the melting point from greater than 330xc2x0 C. to 315-320xc2x0 C. There is also reduction of mechanical strength of the polymer formed. It also leads to a significant reduction in its ability to withstand high processing temperatures of 350-400xc2x0 C. without getting cross-linked. Such a PEEK, therefore, can neither be processed nor be used as a high performance plastic.
Further, the system is highly moisture sensitive due to excess AlCl3 as well as the acid chlorides used as raw materials. Additionally, the precipitation treatment of the reaction mass to liberate the polymer from the catalyst AlCl3 with water involves the liberation of large quantity of HCl gas, which forms effluent. The catalyst AlCl3 used becomes an environmental burden, being non-recyclable and producing hugh quantities of effluents containing Al salts. The process itself is also therefore difficult and inconvenient to carry out with no stringent controls for molecular weights.
Another electrophilic process exemplified by Ueda and Oda uses Methanesulfonic acid (MSA)/Phosphorous pentoxide (P2O3) [JOC 38, 4071, 1973, and Polymer 29, 1903, 1983] at low temperatures like 60xc2x0 C. Inh. V. as high as 1.08 dl/g was obtained. They teach the use of a 1:10 solution by weight of P2O3 in MSA. A mixed anhydride is proposed as the active reagent. While PEEK so produced has less branched structure than one produced using AlCl3 system, it also suffers, like the later, from high temperature instability and hence cannot be molded or extruded without extensive cross-linking and degradation.
Colquhoun has suggested use of Trifluoromethane Sulfonic acid as the reaction medium to polymerize PPBA to give PEEK. (Polymer Preprints, 25, 17, 1984). It has also remained only of academic interest due to the extremely high cost and corrosive nature of the solvent used. Also, in all these above mentioned electrophylic processes, reactive end groups were present. It is part of this invention that PEEK so produced with such reactive groups, like xe2x80x94COOH, present cannot be processed, without end-capping, using traditional plastic processing techniques due to its high thermal instability. Such PEEK on being subjected to high temperature processing immediately cross-links producing gels, which cannot be shaped into desired articles. Therefore, PEEK production by electrophilic processes as described above has not been commercially successful owing to so many inherent limitations involved.
In U.S. Pat. No. 4,247,682 (1981) Dahl has described processes for the condensation of p-phenoxy benzoyl chloride and p-phenoxy benzene sulfonyl chloride in HF using BF3 as a catalyst and using biphenyl or benzoyl chloride as end-capping agents to prepare PEK and PES. These end-capping groups were reported to help maintain the polymer melt stability during extrusion in the absence of which the polymer was reported to degrade readily.
In yet another patent, U.S. Pat. No. 4,808,693 (1989) Dahl, Jansons and Moore have described a process for the condensation of terephthalolyl chloride with Diphenyl ether and diphenoxy benzede using AlCl3/EDC system to yield a copolymer of PEKK and PEEKK. Here too, the authors have highlighted the role of the relative ratio of the two electrophilic agents, where higher diphenoxy benzene quantity has increased thermal stability. It may be assumed that use of a given electrophylic system as well as the monomers employed also played a part in determining final structure giving higher thermal stability of the product.
No mention has been made, by the above mentioned authors or to the best of our knowledge by anyone else, for polymerization of phenoxy phenoxy benzoic acid (PPBA) to yield a melt stable and thermally processible Polyether Ether Ketone (PEEK).
In our studies we have found that not only is the nature of the repeat unit critical for obtaining good thermal and mechanical properties, but the nature of the end-group is also critical for attaining desired thermal stability. By manipulating end-groups, it is now possible to prepare electrophilically, novel PEEK structures which show excellent thermal stability and are therefore inherently melt processible.
The present invention relates to the preparation of novel Polyether Ether Ketone (PEEK) by an improved electrophilic route. The PEEK so made contains un-reactive end-groups making it thermally stable and melt processible. It is hence mouldable and extrudable and useful as a plastic. Further, the process of preparation is not only novel but is very facile and can be made into a manufacturing process. This is because of lower temperature of polymerization, use of readily recyclable reagents, use of water as non-solvent and in general because of overall ease of processing.
The process involves reacting phenoxy benzoic acid (PPBA) in alkane sulfonic acid with a condensing agent, with or without a diluent at 40xc2x0 to 160xc2x0 C. and subsequently endcapping it with a suitable endcapping agent. The alkane sulfonic acid used can be any aliphatic sulfonic acid including haloalkane sulfonic acid, preferably Methane Sulfonic acid (MSA), Trifluoro or Trichloro Methane Sulfonic acid. The condensing agents used for example are Thionyl Chloride, Phosphorous trichloride, Phosphorous pentachloride or Phosphorous pentoxide, Methane Sulfonic Anhydride or their mixtures. The diluent is, for example, a non-polar aprotic solvent such as Methylene Chloride, Ethylene Dichloride or Sulfolane, or their mixtures or any of the polar organic compounds remaining inert in this system. The end-capping agent is an aromatic compound like Benzene, Toluene, Xylene, Phenol, Anisole, Diphenyl Ether or any of their stable derivatives.
The alkane sulfonic acid mixture with its anhydride was found to be a solvent as well as a powerful catalyst for polymerization, of phenoxy phenoxy benzoic acid (PPBA) to give PEEK. It was also found that in MSA-MSAN system, the Methane Sulfonic Acid Anhydride (MSAN), gets easily reconverted into MSA after reaction work-up in water, so that recycling is possible and waste generation is minimal, MSA being recoverable and recyclable. Additional, the MSAN need not be prepared separately and added during the reaction, but can also be prepared in-situ. When the reaction mass is added into water during the work-up of the reaction, the acid remains dissolved in water and the polymer easily precipitates out. The polymer is isolated by filtration, and is washed and dried. Any unreacted anhydride present in the reaction mass gets reconverted into the acid by reaction with water during the work-up. The resultant acid and water mixture can be easily separated by fractional distillation and both the acid and water can be recycled for the next batch.
In MSA-P2O3 system, similarly, MSA and H3PO4 are formed on precipitation of PEEK in water, from which MSA can be separated and reused. Thus, a significant advantage of our process is the ease of work-up and isolation of polymer due to the use of aqueous system for precipitation of PEEK. Further purification of PEEK is required to remove traces of acid, which can be accomplished by washing with hot water containing alkali and refluxing in water again to remove last traces of salt and alkali.
Generation of MSAN is readily carried out with any of the condensing agents. Thionyl Chloride (SOCl2) or Phosphorous Pentoxide are the preferred reagents, due again to the ease of operation and feasibility of using the by-products, with minimum waste generation.
SOCl2 reacts with MSA to give flue gasses SO2 and HCl, which can be reacted back to give SOCl2, and recycled, (Geiko V. I., Gladushko et al. Khim. Khim. Tecknol. 1985, 28(5)-4 (Russ)). With P2O3, similar treatment yields H3PO4, a useful acid, which also can be separated from its mixture with MSA by extraction with suitable solvents or by fractional distillation of MSA and used as such. Such recycling was not achievable when a common catalyst like AlCl3 was used for polmerization.
The invention of this process makes it possible to prepare PEEK electrophilically using homogenous solutions. The old AlCl3 process was heterogeneous making molecular weight control nearly impossible as PEEK tended to crystallise out of solution forming a slurry. In the present process, rate of polymerization reaction and ultimate molecular weights can be readily controlled by a proper choice of reaction, temperature, monomer concentration, and the quantity of anhydride employed. Thus, the kinetics of polymerization is easily controlled. Another advantage is the use of an acid monomer as the precursor, instead of the acid chloride as the precursor in case of manufacture based on Freidel-crafts synthesis with catalysts, like AlCl3. As is widely known, an acid chloride is susceptible to hydrolysis, even in presence of trace quantities of water. These problems are overcome here by using acid monomer itself.
Another novel and important part of this invention is the end capping used for the removal of reactive end groups of PEEK. It has been shown using FTIR spectra that the PEEK chains contain a carboxyl group, xe2x80x94COOH, at one end. Such a group is known to be a reactive group, particularly at high processing temperatures of PEEK. It can thus lead to formation of cross-links or undergo other reactions disturbing rheology. To protect the PEEK produced by our method, a novel end capping process was carried out. The reaction mass was diluted with Toluene, after the desired Inherent Viscosity was attained. Over a period of a few hours, Toluene reacted with the chain end and formed a keto group, which shows low reactivity and high thermal stability unlike the carboxyl group. Other reagent like Benzene, Xylene, Phenol, Anisole, Diphenyl Ether etc. were also successfully used for end capping. This end capping is important to attain high thermal stability as will become evident from the examples. Thus, overall novel PEEK structures were developed which are melt processible exhibiting good thermal stability over extended periods of time at high temperatures of PEEK processing.
Another novel part of the invention is use of diluents such as CH2Cl2, Dichloroethane, Sulfolane, etc. Since the polymerization temperatures are low to moderate in the range of 60-100xc2x0 C. only, towards the end, the reaction mass viscosity increases and it makes efficient stirring difficult. Addition of diluents, like CH2Cl2 or CH2Clxe2x80x94CH2Cl or Toluene, helps in keeping the solution stirrable and improves mixing. Another advantage derived by the addition of a diluent, which can be low boiling and immiscible with water, is that on precipitation in hot water, it simply boils off and thus gets readily separated. Another advantage of adding a diluent is that the precipitating PEEK is obtained in this case as fine granules or powder. Without such a diluent, lumps or thick fibres are obtained requiring further size reduction. Yet another advantage of using the diluent is that the PEEK obtained as powder contains less than 10% MSA entrapped in it, while the lumpy or fibrous PEEK contains as much as 15-25% MSA entrapped, requiring more exhaustive post-polymerization treatments.
After separation from the reaction mass by precipitation in water, the polymer is filtered and washed conveniently free of MSA and H3PO4, if any, as shown in the examples. PEEK is subsequently treated in refluxed water, followed by refluxing in alkaline solution. Alternately, an organic base like Dimethyl Formamide (DMF) or Dimethyl Acetamide (DMAc), etc. can also be used. A Formic acid treatment is optionally given to PEEK samples with higher UV absorbance or high As values, to reduce them to lower As levels required by our invention and to improve its color from buff to white during powder stage.
It is a part of this invention that PEEK so produced has to be made completely free of the solvent, as even small quantities of the solvent left behind has very deleterious effect on the processability of PEEK at high temperatures.
Thus, PEEK by the above process can be prepared under controlled moderate to high molecular weights as shown by its Inh. V. and Gel Permeation Chromatography (GPC) molecular weights. It also has a controlled structure as shown by its solutions having moderate to low absorbance in UV spectra. As against an absorbance value, As, of less than 20 for linear nucleophilic PEEK, PEEK prepared by this method as As values in the range of 300-600. It has been postulated hat this As value may possibly indicate presence of branch structure. However, no direct proof has yet been found to confirm such a postulate of branches on the backbone. However, this appears to have no influence on processibilty when the reactive ends of the polymer are suitably capped. The reactive groups on PEEK of this invention can be suitably reacted to end cap the polymer. All these make it possible to obtain high thermal stability and also processibility for such PEEK. It is therefore understood that PEEK structures of our invention are essentially linear polymers with novel end groups.
Hence, an object of this invention is to provide novel Polyaryl Ether Ether Ketone (PEEK), prepared using electrophilic process but which is thermally stable and melt processible.
Another object of this invention is to provide PEEK of novel structure, which contains controlled negligible number of branches on the backbone and whose reactive end groups are capped.
Another object of the invention is to provide a process for the preparation of thermoplastic PEEK, which is carried out at low temperatures like 40xc2x0-100xc2x0 C. and is therefore economically more attractive for commercial production. This process not only produces PEEK of high enough Inh. V. to be of practical interest but also helps in reduction of it""s defects arising out of branch points and reactive end groups present on the backbone to make it thermally stable for high temperature processing and usage.
Another object of this invention is to provide a process for the production of PEEK, which minimises waste generation, due to the possibility of recycling its reactants.
According to the invention, there is provided a process for the production of a novel aromatic Polyaryl Ether Ether Ketone (PEEK), which involves polymerising Phenoxy phenoxy benzoic acid (PPBA) using alkyl sulfonic acid and a condensing agent with or without a diluent at 40-100xc2x0 C. and subsequently end capping it using an end capping agent. Thereafter, separating PEEK from the reaction mixture by precipitation in water and giving further water treatments for purification. Further, treating it with organic solvent with or without Formic acid to improve the colour of the PEEK powder is also part of this invention.
The specific examples that follow will serve to illustrate the invention but should not be construed to limit the scope thereof.